The present invention relates generally to an apparatus and method for measuring a surface position of an object, such as, though not exclusively, a wafer for a semiconductor device and a glass plate for a liquid crystal display (“LCD”). And the present invention is applicable, for example, to an exposure apparatus used in a lithography process for manufacturing a device, such as a semiconductor device, a LCD device and a thin-film magnetic head.
In manufacturing devices using the photolithography technology, a projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask (reticle) onto a wafer, etc. to transfer the circuit pattern.
The projection exposure apparatus is required to expose the reticle's circuit pattern onto the wafer with high resolution for finer and higher-density integrated circuits (“ICs”). The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. The recent light source has shifted from an ultra-high pressure mercury lamp (such as g-line with a wavelength of approximately 436 nm and i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). An F2 laser (with a wavelength of approximately 157 nm) is about to reduce to practice. A further expansion of an exposure area is demanded.
A conventionally used step-and-repeat exposure apparatus (also referred to as a “stepper”) exposes a reduced size of an approximately square exposure area onto a wafer. On the other hand, a step-and-repeat exposure apparatus (also referred to as to a “scanner”), which is suitable for precise exposure of a large screen and thus is about to become the main current, sets a rectangular or arc-shaped slit that is part of the exposure area and relatively scans the reticle and the wafer at a high speed.
In exposure, the scanner uses a surface-position detecting means of an oblique light projection system to measure a surface position of part of the wafer before the exposure slit area moves to the part of the wafer, and accords the part's surface with the best exposure image-plane position in exposing the part, thereby reducing influence of the wafer's flatness. In particular, plural measurement points are set in longitudinal direction of the exposure slit, i.e., a direction orthogonal to the scan direction, to measure the tilt of the wafer surface. Japanese Patent Application, Publication No. 6-260391, for example, discloses a wafer surface position measuring method for the scanner.
A brief description will be given of an exposure apparatus configuration that has a conventional wafer surface shape or position measuring apparatus. FIG. 13 is a block diagram of a schematic structure of the conventional exposure apparatus. The exposure light emitted from a light source 800 that uses, for example, an excimer laser, is shaped into an exposure slit having a predetermined shape suitable for exposure, and illuminates a patterned surface of the reticle 101. The patterned surface has a circuit pattern to be exposed, and the light that passes the circuit pattern forms a circuit pattern image on the imaging surface via a projection lens 102. A wafer 103 surface is located near the imaging surface.
The reticle 101 is held on a reticle stage RS that scans the reticle 101 in a Y direction. A wafer stage WS holds the wafer 103 and drives the wafer 103 in XYZ directions and around XYZ axes.
The circuit pattern of the reticle 101 is transferred onto a shot area as an area to be exposed on the wafer 103 by scanning the reticle stage RS and the wafer stage WS at a speed ratio corresponding to the exposure magnification. After exposure to one shot area (one shot exposure) ends, the wafer stage WS steps the wafer 103 to the next shot area for the scanning exposure in the −Y direction, inverse to the just previous scanning direction. A series of these actions is referred to as a step-and-scan exposure method unique to the scanner. The step-and-scan action exposes all the shot areas on the wafer 103.
During scanning in the one shot exposure, a focus and tilt detection system 133 obtains surface position information of the wafer 103 surface, and calculates a shift amount from the exposure image plane. The stage actions in the Z and tilt directions correct a position on the wafer 103 surface at almost an exposure slit unit. FIG. 14 shows a schematic structure of the focus and tilt detection system 133, but a detailed description thereof will be omitted because details of this structure is disclosed in Japanese Patent Application, Publication No. 6-260391.
The focus and tilt detection system 133 optically measures a height of the wafer 103 surface. The light is incident at a highly incident angle upon the wafer 103 surface, more particularly, the resist surface applied onto the wafer 103, and a position detector, such as a CCD, detects a shift of a reflected light image. The light is incident upon plural measuring points on the wafer 103, and the reflected light is guided to a separate position detector to detect the height of the wafer 103 surface at a different position. Based on the measuring result, a position of the wafer 103 is corrected in the Z direction and/or tilt direction (in a rotational direction around the X-axis direction and/or in a rotational direction around the Y-axis).
For the finer and higher-density ICs, the drastically reduced depth of focus (“DOC”) of the exposure optical system narrows latitude in aligning an exposed wafer surface with the best focus position or requires stricter focus precision. As a result, the influence of a pattern formed on the wafer and a measuring error of the surface position detection system caused by the uneven resist thickness cannot become negligible.
FIG. 9 is an explanatory view for explaining reflectance changes associated with the resist thickness changes due to the patterned steps on the wafer. The reflectance on the resist applied wafer depends upon the influence between the reflected light on the resist front surface and the reflected light on the resist back surface (or wafer pattern front surface). Since a resist thickness Rt′ in a step part B is greater than a resist thickness Rt in a non-stepped area A on the wafer, an optical-path difference dA between reflected light ka1 on the resist front surface and reflected light ka2 on the resist back surface with respect to the light incident upon the area A is different from an optical-path difference dB between reflected light kb1 on the resist front surface and reflected light kb2 on the resist back surface with respect to the light incident upon the area B. As a result, the reflectance differs between the areas A and B. An asymmetrical signal waveform occurs when a projected image or measuring light of a grating pattern on a patterned plate is incident upon the areas having different reflectance changes. This reflectance changes occur not only in the simple resist thickness changes as in FIG. 9, but also another cause as in FIG. 10. FIG. 10 is a view for explaining a reflectance difference between an area C having no pattern or a low pattern density and an area D having a high pattern density. The resist thickness is equal between the areas C and D, and reflected lights kc1 and kd1 have almost the same reflectance on the resist front surface between the areas C and D. However, the areas C and D have different pattern densities, and reflected lights kc2 and kd2 have different reflectances on the resist back surface. In addition, when the wafer pattern structure is smaller than the wavelength of the illumination light, a phase jump phenomenon of the reflected light occurs, which is referred to as structural birefringence, and causes a phase difference between the reflected lights kc2 and kd2 on the resist back surface. Thereby, the areas C and D have different reflectances.
Since the reflection angle and reflected light intensity thus change according to wafer patterns, the detected waveform originating from the received reflected light contains asymmetry, consequently causing a detection error, remarkably lowers the contrast of the detection waveform, and hindering a precise detection of a surface position. In particular, for a wafer pattern size smaller than 65 nm, the focus measuring accuracy should be maintained lower than several nanometers, and the current optical measurement alone has a difficulty in maintaining the sufficient focus precision.